Vinyiidene fluoride polymers, thanks to their partially fluorinated backbone, have been long known and have been used in a variety of applications because of several desirable properties such as mechanical properties, chemical resistance, thermal resistance, etc.
A frequently used method for producing VDF polymers involves aqueous emulsion polymerization of VDF with optionally one or more fluorinated monomers generally involving the use of fluorinated surfactants.
Frequently used fluorinated surfactants include perfluoroalkyl carboxylic acids and salts thereof. Nevertheless, due to certain environmental concerns, efforts are now devoted to phasing out from such compounds and methods wherein VDF polymer products are manufactured using alternative surfactants having a more favourable toxicological profile are attracting increased attention.
Among polymerization techniques known in the art, a method for manufacturing thermoplastic VDF polymers, wherein certain partially fluorinated surfactants of general formula Rf—C2H4—SO3M, wherein Rf is a perfluorinated radical having 4 to 10 carbon atoms and M is an alkali metal such as lithium, sodium or potassium or an ammonium radical, are used, is known from U.S. Pat. No. 4,025,709 (UGINE KUHLMANN) 24.05.1977. In this document, it is taught that said surfactant should be used in amounts of between 0.2 to 0.5% based on weight of water (2 to 5 g/l).
Nevertheless, VDF polymers obtained from such polymerization process undergo substantial yellowing/darkening phenomena, in particular when exposed to temperature of 260° C. or more, as an evidence of thermal degradation issues.
On the other side, similar fluorosurfactants have been used in the past for the manufacture of fluoroelastomers.
Thus, U.S. Pat. No. 4,524,197 (DUPONT DE NEMOURS) 18 Jun. 1985 discloses a process for manufacturing VDF-based fluoroelastomers in the presence of surfactants of formula: F—(CF2CF2)n—CH2CH2OSO3M, with n=2-8 and M is an alkali metal cation, hydrogen or ammonium. These sulphate surfactants are nevertheless endowed with active tension properties which are less advantageous over corresponding sulphonic counterparts.
Finally, U.S. Pat. No. 6,774,164 (DUPONT DOW ELASTOMERS L.L.C) Oct. 8, 2004 discloses an emulsion polymerization process for the production of fluoroelastomers in the presence of a surfactant of formula F—(CF2CF2)n—CH2CH2—SO3M, with n from 2 to 9, and M being a cation of valence of 1, said process being initiated by water-soluble peroxides including ammonium, sodium or potassium salts of hydrogen persulfate.
Nevertheless, polymerization techniques suitable for fluoroelastomers are not directly applicable to the manufacture of thermoplastic VDF polymers, which possess different colloidal behaviour and completely different properties.
Further, in addition, in processes of the prior art, inorganic initiators have been used which, while known for positively affecting colloidal stability of lattices because of the formation of ionic end groups on growing chains, nevertheless lead to products having, because of these very same end-groups, moderated thermal stability, and which are prone to undergo to discoloration phenomena.
There is thus still a shortfall in the art for a method for emulsion polymerizing VDF polymers, which can provide with high conversions, with substantial no reactor build-up nor coagulation losses, thermoplastic VDF polymers having outstanding thermal stability and high molecular weight.